1. Technical Field of the Invention
The present invention relates to a detection apparatus, and in particular to a detection apparatus that detects an amount of particulate matter in an exhaust gas that flows through the exhaust path of an internal combustion engine.
2. Related Art
Recently, internal combustion engines are required to have superior exhaust purification performance. In diesel engines, in particular, removal of so-called exhaust particulates (particulate matter (PM)), such as black smoke, exhausted from the engines is of increasing importance. In order to remove PM, diesel engines are most commonly equipped with a diesel particulate filter (DPF) in the middle of the exhaust pipe.
PM sensors are one of the means for detecting the amount of PM in an exhaust gas. For example, using a detection value derived from a PM sensor disposed downstream of a DPF, a failure of the DPF, if any, can be detected. Further, when such a PM sensor is disposed upstream of a DPF, the amount of PM accumulated in the DPF can be estimated from a detection value derived from the PM sensor. For example, JP-A-559-060018 discloses a system for estimating the amount of PM accumulated in a DPF by disposing a PM sensor in an exhaust pipe.
As shown in FIG. 18, a PM sensor 5 of a typical structure includes an insulator 50, a pair of electrodes 51 and 52, and a power supply 54. When the PM sensor 5 is disposed in an exhaust pipe through which PM flows, PM is deposited on the insulator 50. Since PM is electrically conductive, accumulation of PM between the electrodes 51 and 52 to the extent of connecting therebetween will create an electrically conductive state across the electrodes. Accordingly, when voltage is applied by the power supply 54 across the electrodes 51 and 52, current passes across the electrodes 51 and 52. As more PM is accumulated between the electrodes 51 and 52, more current passes across the electrodes. Therefore, the amount of PM accumulated on the insulator, and further, the amount of PM in the exhaust pipe is detected (estimated) based on the current passing across the electrodes.
The use of the PM sensor is required to burn PM attaching to the PM sensor so as to regenerate the PM sensor each time an amount of PM attached (deposited) to the PM sensor (its insulator) is judged to be too large. FIG. 17 shows an example of this case.
As shown in FIG. 17, after a completion of a regeneration process of the PM sensor, an amount of PM attaching to the insulator increases from zero state with time, but an output value of the PM sensor remains in a zero state until a positive electrode and a negative electrode (corresponding to the electrodes 51 and 52 in FIG. 18) are electrically connected via PM deposited. At one point, once the positive electrode and the negative electrode are electrically connected, the output value of the PM sensor starts to increase. If the output value of the PM sensor exceeds a predetermined threshold level, the regeneration process is performed. The above processes are repeated during operation of an engine.
In the regeneration of the PM sensor, if a regeneration period is too short, a part of PM may remain after burning to thereby reduce accuracy of detecting the amount of PM. On the other hand, for example, if the regeneration period is too long, a failure of the DPF cannot be detected during the regeneration of the PM sensor. Therefore, an unnecessarily long length of the regeneration period is required to be avoided.
A temperature (electrode temperature) needed to burn PM during the regeneration of the PM sensor is controlled to follow a set target temperature. If the target temperature is too high, PM attaching to the PM sensor rapidly burns and the PM sensor may be damaged. In contrast, if the target temperature is too low, it takes a long time to burn PM and then a long regeneration period of the PM sensor is required. This is not desirable. Therefore, the target temperature is required to be properly set. In the related art, above-mentioned situations, where the length of the regeneration period and the target temperature are required to be properly set during the regeneration of the PM sensor, are not recognized as problem to be solved.